JPH02308129A - Ferroelectric liquid crystal element and driving method thereof - Google Patents

Abstract

PURPOSE:To allow black and white display without coloration by forming oriented films of an inorg. insulator which are reverse in inclination direction from each other on the opposite surfaces of a pair of substrates and specifying the thickness of the liquid crystal layer thereof to a prescribed range. CONSTITUTION:Two sheets of the glass substrates 11 constituted by coating the surfaces of transparent electrodes 14 with inorg. insulating films 13 having a high dielectric constant, then forming the oriented films 12 of SiO formed by a diagonal vapor deposition method are so combined that the respective oblique oriented layers of the SiO are reverse inclined. In addition, the liquid crystal cell is so constituted that the spacing between the oblique oriented layers of the SiO attains 1.5 to 4.0mu. A liquid crystal compsn. mixture contg. a ferroelectric liquid crystal of an ester system which is liable to be oriented in the oblique oriented layer of the SiO is injected in this cell. Two sheets of polarizing plates are disposed on the upper and lower glass substrates in such a manner that the transmittance is lowest when a positive or negative voltage is impressed to the element. The ferroelectric liquid crystal element is thus obtd. The black and white display which is substantially free from the coloration is possible in this way.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal device, and more particularly to a ferroelectric liquid crystal device that has both high-speed response and memory performance, and a method for driving the same.

[Conventional technology and issues]

Ferroelectric liquid crystal elements are expected to be put to practical use in high-capacity displays and high-speed liquid crystal shutters for printers, taking advantage of their memory properties and high-speed response.

Many of the ferroelectric liquid crystal devices that have been prototyped to date are
Alignment film with rubbed organic polymer film on the opposing surface or Si
This is a liquid crystal element in which a ferroelectric liquid crystal is sandwiched between two glass substrates having an alignment film formed by oblique evaporation of an inorganic insulator such as oxygen. For example, Japanese Unexamined Patent Publication No. 173433/1983 discloses a liquid crystal element using a polyimide polymer thin film subjected to a rubbing treatment. However, this liquid crystal element has a uniform liquid crystal alignment state (third state) when voltage is applied.
When the drive voltage is returned to zero (state (a) or (b) in the figure), the uniform arrangement collapses, and the liquid crystal molecules become splayed (schist) arranged with a twisted structure, resulting in very poor memory performance. be. On the other hand, JP-A-62
In the method 5 shown in Publication No. 250418, S
A liquid crystal element configured with two substrates to which alignment films formed by obliquely deposited iO films are arranged in a forward tilted state 5 exhibits a memory effect, but memory unevenness occurs depending on the location, and uniform switching cannot be obtained. It has some serious drawbacks.

On the other hand, JP-A-62-160420 discloses a liquid crystal element in which liquid crystal molecules have three stable states: two uniform states and a spray state.
It has been confirmed that the display mode in the spray state has less display unevenness due to memory unevenness, etc. However, the structure and driving method of a liquid crystal element that provides three stable display states has not been obtained.

Another problem that arises when putting ferroelectric liquid crystal elements into practical use is that the bright state of ferroelectric liquid crystal elements is colored in various ways depending on the refractive index anisotropy of liquid crystal molecules and the thickness of the liquid crystal layer. However, ferroelectric liquid crystal devices have the disadvantage that they are difficult to display in black and white.

Accordingly, an object of the present invention is to provide a ferroelectric liquid crystal element that has excellent memory effects and uniformity, and provides a bright state closer to white, and a method for driving the same.

[Means to solve the problem]

In order to achieve the above object, the present invention devises a new structure and driving method for a liquid crystal element having the above-mentioned splay alignment state as one stable state.

First, in order to realize a liquid crystal element with a stable splayed alignment state and two uniform alignment states, a pair of substrates having transparent electrodes on their opposing faces were formed by oblique vapor deposition on the opposing faces of each substrate, and It is characterized by forming alignment films of inorganic insulators with mutually opposite tilt directions, sandwiching a ferroelectric liquid crystal between the substrates, and changing the thickness of the liquid crystal layer from 1.5 microns to 4 microns. It is said that

Furthermore, using one of the above-mentioned splay alignment state and two uniform alignment states of the liquid crystal element as two stable states,
It is characterized by performing binary electro-optical control.

In addition, as a method for driving a liquid crystal element that performs binary electro-optical control, two uniform arrays are placed between transparent electrodes in order to select the uniform array state among the two stable states for binary control. When setting the uniform array state to apply a pair of positive and negative pulses of varying magnitude from one state to the other to set the splay array state, which is one of the two stable states for binary control. It is characterized by driving by first applying the same positive and negative pulses, and then continuously applying one or more pairs of pulses whose product of voltage value and pulse width gradually decreases to the extent that the uniform state of the other does not change. .

[Effect]

In a liquid crystal element in which a ferroelectric liquid crystal is sandwiched between two glass substrates to which obliquely vapor-deposited films of an inorganic insulating material are attached with opposite tilt directions, the liquid crystal alignment state differs depending on the thickness of the liquid crystal layer. Figure 3 is a schematic diagram showing the possible liquid crystal alignment states of liquid crystal molecules in a ferroelectric liquid crystal element. A certain liquid crystal molecule 61 is marked with an O mark, and a liquid crystal molecule 6 on the opposite side is marked with an O mark.
2 is marked with a 0 mark, and the liquid crystal molecules in the middle are not marked. Moreover, 64 is a glass substrate with a transparent electrode to which the obliquely evaporated alignment film 36 is attached. In this figure, (a) shows a uniform array of 61 molecules, with only the vicinity of the substrate interface being arrayed counterclockwise; (
b) shows a uniform arrangement of 62 molecules, with a clockwise arrangement only near the substrate interface, and (C) shows a mixed state of a counterclockwise splay arrangement and a clockwise splay arrangement. Experimental results have revealed that the stable states of states (a), (b), and (C) differ depending on the thickness of the liquid crystal layer. That is, when the liquid crystal layer thickness is 4μ or more, (a) or (b) is applied when a positive or negative voltage is applied.
Once in the state of , the liquid crystal molecules align, but when the voltage is returned to zero (
Only state C) tends to be stable. On the other hand, if the liquid crystal layer thickness is set to 1.5μ or less, both states (a) and (b) will maintain the state when voltage is applied even when the voltage is returned to zero, indicating a so-called bistable state, but the stability This is more likely to occur than the bias. That is, the magnitude of the positive or negative write voltage that causes state (a) and state (b) varies greatly depending on the location, resulting in uneven memory characteristics. By the way, the liquid crystal layer thickness is 4
According to the author's experimental results, in the range from μ to 1.5 μ, three stable states exist independently: 61), (b), (C), and 0. State (C) is a uniform arrangement state (a
) and (b), state (C) can be easily obtained by attenuating the voltage peak value or pulse width while switching between (a) and (b) by applying positive and negative voltages. This state is maintained semi-permanently unless voltage is applied thereafter. Further, due to the existence of the intermediate state (C), the write voltages of the uniform array states (a) and (b) have approximately the same value, and the unevenness of the memory characteristics is rapidly reduced. To summarize the above-mentioned changes in the liquid crystal alignment state depending on the liquid crystal layer thickness, at a thickness of 4μ or more,
It is likely to be in a monostable state with only the Carex violet array state; 1.
At a thickness in the range of 5μ to 4μ, three stable states are shown: two uniform alignment states and a snail alignment state.If the liquid crystal layer thickness is further reduced to 1.5μ or less, only two uniform alignment states become stable; The characteristics differ depending on the location, resulting in an asymmetry in memory characteristics. On the other hand, when the ferroelectric liquid crystal element is switched using the splay alignment state and the uniform alignment state (5), the coloring in the bright state is reduced compared to the conventional switching between the uniform alignment states, resulting in a clear black and white display. This is because the optical properties of the splay alignment state are different from the uniform alignment state in which the upper and lower substrates are aligned in one direction, but the liquid crystal molecules are arranged in a twisted manner, so when light passes through the liquid crystal element, it undergoes optical rotation. This is to do so. As shown in FIG. 4, which schematically shows how the liquid crystal molecules are arranged from the top of the liquid crystal element, the upper and lower substrates are aligned so that the polarization axis 44 under crossed Nicols matches the orientation of the liquid crystal molecules 41 in one uniform alignment state. When a polarizing plate is placed above, a dark state occurs in the alignment state of the liquid crystal molecules 41, and a black level of a polarizing plate cross is obtained. ° On the other hand, liquid crystal molecules 42
The bright state that occurs in the uniform alignment state is colored, and the transmitted light T is expressed by equation (1) as a function of the refractive index anisotropy Δn of the liquid crystal, the liquid crystal layer thickness d, and the wavelength λ of the transmitted light.

T = sin” (πΔnd/λ) ・
(11) For typical ferroelectric liquid crystals with Δn values of 0.15, coloration becomes intense when d is 2μ or more, with yellow at 2.5μ, red at 3μ, and sky blue at 4μ. The coloring in the spray alignment state in which the liquid crystal molecules 46 in FIG.
．． At 5μ it becomes white, at 3μ it becomes IJ-color, and at 4μ it becomes pale yellow.If the liquid crystal layer thickness is at least about 4μ, switching between the splay alignment state and the uniform alignment state allows black and white display with almost no coloring. A ferroelectric liquid crystal element capable of this can be obtained.

〔Example〕

The present invention will be explained below with reference to Examples.

(Example 1) Fig. 1 shows a ferroelectric liquid crystal device showing an example of the present invention, and the figure shows its configuration. It represents the transmittance of the state. A detailed explanation will be given below based on FIG. 1. Iゎ20 of 1200A in Figure 1 Prisoner. On the transparent electrode 14, 500A Zγ-8 made by the alkoxide method
After coating with an iO-based high dielectric constant inorganic insulating film 16, a 700A SiO alignment film 1 was created by oblique evaporation method.
The two glass substrates 11 on which Si
5 so that the O tilted orientation layer is reversely tilted, and S
A liquid crystal cell is constructed such that the gap between the iO inclined alignment layers is 1.5 μ to 4.0 μ. A mixed liquid crystal composition containing 40% or more by weight of ester-based ferroelectric liquid crystal, which is easily aligned with a SiO inclined alignment layer, is injected into the above liquid crystal cell, and a positive or negative voltage is applied to the element using two polarizing plates of a crossed niffle. A ferroelectric liquid crystal element is formed by disposing the upper and lower glass substrates so that the transmittance decreases the most when the voltage is applied. The operation mode is shown in Figure 1 (8 shows the pulse width of 1 ms) using a ferroelectric liquid crystal composition C8-1014 manufactured by Chisso as an example under the condition that the liquid crystal layer thickness is 3.0μ.
The wave height values are -2V, 2■, respectively. (However, v0=i
A uniform arrangement state (a) of reddish transmitted light is written by the positive and negative symmetrical voltage waveform of sv), and the pulse width is 2Vo, -2V.

The symmetrical voltage waveform of the black level uniform array state (
b) is obtained, and furthermore, the voltage peak value is 2 with the same pulse width.
VO, -2VO, Vo, -Vo are attenuated and the symmetrical voltage waveform shows a nearly white splay array state of transmitted light (C
) is obtained. Here, when the voltage is 2V, =30V, if the peak value of the second positive and negative pulses is 5 to 20V, the state changes to a spray array state. These three states (a), (b), and (C) are stable semi-permanently unless a new voltage is applied, and there is no variation depending on the location of the converted voltage between each state (good switching is possible). According to the author's experiments, the liquid crystal layer thickness at which states (a), (b), and (C) stably appear evenly is the same when using more than 10 mixed liquid crystal materials containing ester-based ferroelectric liquid crystals. The result was 1.5μ to 4.0μ.
Preferably, each state showed no unevenness (or almost the same stable state) when the liquid crystal layer thickness was from 2 μm to 3.5 μm.

(Example 2) Example 1 A ferroelectric liquid crystal element with a finch screen size having 128 scanning electrode groups and 160 signal electrode groups was prepared.
An actual driving display was attempted using the C8-1014 described above under the condition that the liquid crystal layer thickness was 3.0 μm.

As shown in FIG. 2, the driving waveform is the scan electrode voltage waveform (T
s, Tt, Ts...) are applied with two pairs of positive and negative symmetrical voltage waveforms in which the voltage peak value and pulse width decrease,
A symmetrical voltage waveform in the order of negative and positive is applied to the signal electrodes (Ss, St...) when writing the white state, and conversely, a symmetrical voltage waveform in the order of positive and negative is applied when writing the hot state. ing. The pixel T, -8° selected in the white state is once in a uniform array thermal state written with a positive and negative symmetrical voltage waveform with a peak value of 20v0 and a pulse width of 2to,
Next, at a peak value of 5V0 that exceeds the threshold value for changing to the spray state, it is converted to the white state of the spray array with a positive and negative symmetrical voltage waveform of a pulse width of to, and this state is not disturbed by the non-selected bias voltage of ±vo. It remains in the white state. On the other hand, the pixel T, -8t selected in the thermal state is ±2QV
Even after being converted to black level by the voltage waveform of 0, ±3V
With a voltage waveform of 0, pixel T, -8, is maintained at the black level without disturbing the uniform black state. Especially V
, = 1.5 V, and t = 4 m s. As a result, a black and white display with a contrast ratio of 10, which was impossible with conventional switching between uniform states with a liquid crystal layer thickness of 3 μm, was obtained.

〔Effect of the invention〕

As described in the above embodiments, the ferroelectric liquid crystal element of the present invention, which can take three stable states and has a liquid crystal layer thickness of 1.5 to 4.0 μ, has uniform memory characteristics and is capable of good switching. Furthermore, in a ferroelectric liquid crystal display element having a liquid crystal layer thickness of 2 μm or more, it is possible to realize a black and white display, which has been difficult to achieve in the past.

[Brief explanation of drawings]

Figure 1 (to) is a schematic diagram showing the configuration of the ferroelectric liquid crystal element of the present invention. Figure 2 is an explanatory diagram showing the relationship between crystal molecular alignment and polarizing plate arrangement of the present invention. CB) Figure 3 (Q) (b) (c) (d)

Claims (3)

[Claims] (1) A ferroelectric liquid crystal in which a ferroelectric liquid crystal is sandwiched between a pair of substrates having transparent electrodes on opposing surfaces, and the ferroelectric liquid crystal can take three stable states: a splay alignment state and two uniform alignment states. In the device, an alignment film of an inorganic insulator is formed on the opposing surfaces of the pair of substrates by oblique evaporation so as to be inclined with respect to each surface, and the directions of inclination are opposite to each other, and the thickness of the liquid crystal layer between the substrates is 1. A ferroelectric liquid crystal element characterized in that the ferroelectric liquid crystal element is made from 1.5 microns to 4 microns so that the three stable states described above can be taken. (2) The ferroelectric liquid crystal element according to claim 1, wherein binary electro-optical control is performed by one of two stable states: a splay alignment state and one of two uniform alignment states. (3) Applying a pair of positive and negative pulses of different magnitudes from one side of the two uniform array states to the other between the transparent electrodes,
After setting one of the uniform array states in claim 2 and applying the pair of positive and negative pulses between the transparent electrodes, the product of the voltage value and the pulse width is continuously applied stepwise to the extent that the other uniform array state remains the same. 3. The method of driving a ferroelectric liquid crystal element according to claim 2, wherein a pair of positive and negative pulses are applied to reduce the value of the ferroelectric liquid crystal element to set the ferroelectric liquid crystal element in a splay alignment state.